Liquid photopolymers of paste like consistency are in use for photopatterning coatings on printed wiring boards as evidenced by my U.S. Pat. No. 4,424,089, Jan. 3, 1984 for Photoprinting Process and Apparatus for Exposing Pasteconsistency Photopolymers. In that art the photopattern image bearing surface is pressed into contact with the polymer to produce high resolution coating patterns with non-collimated radiation of low intensity permissible because the polymer does not have its outer surface exposed to air. It is important to keep heat down and photospeed up in this manner.
Liquid polymers are imaged in U.S. Pat. No. 4,436,806, F. J. Rendulic, et al., Mar. 13, 1984 with the outer surface exposed to air thus requiring greater exposure energy and generating considerable heat. Furthermore to obtain high resolution coating patterns, collimated radiation is required to eliminate the noise from scattering at the interfaces and particularly from the air gap interface.
When the liquid polymer layers are superimposed over irregular surfaces, such as those encountered on printed wiring boards having conductor traces extending about 0.001 inch above the substrate surface, they have the significant advantage of covering the rough surface without entrapment of air, which would lower resolution and adversely affect curing and adhesion.
However when the phototool with image, generally a flexible film layer, is pressed into place into intimate air free contact with the liquid polymer layer superimposed over the wiring traces, then the liquid polymer tends to be displaced from the wiring traces and thinned or starved. Thus, the following photopatterning step will leave a deficient thickness of covering over the wiring traces. When solder masking is required, for example, a layer of substantial thickness is required over the conductors for proper adhesion, insulation and resistance to the heat and stresses of soldering. This problem is addressed by this invention.
Another problem introduced by the use of non-collimated radiation is the inconsistency with high resolution patterns. Thus surface interfaces, such as between a liquid polymer layer and a plastic film carrying the desired image pattern, can cause reflections of the radiant energy which introduces noise tending to polymerize the polymer in unwanted places. Thus in printed wiring boards undercutting may occur to cause reduced adhesion, short circuits, open circuits or the like. This has been resolved by using collimated radiation for photoexposure, which is expensive.
It is therefore a general object of this invention to resolve the problems of the prior art as set forth hereinbefore ato obtain high resolution photopatterning of liquid polymers using uncollimated radiation.
It is a further object of the invention to provide a method of producing superior products with high resolution photoimaged patterns produced with liquid photopolymers, with low photo energy and resulting heat while resolving problems of flow of the liquid polymer off of roughly contoured surface peaks, such as produced by conductor traces superimposed on the surface of a printed wiring board substrate.
Another general object of the invention is to produce a cured polymer layer in its permanent substrate position which has a tacky surface for further processing.